Convert resources to take an instance key instead of an Instance*.

ppapi/cpp/dev/file_chooser_dev.h

Index: ppapi/cpp/dev/file_chooser_dev.h
diff --git a/ppapi/cpp/dev/file_chooser_dev.h b/ppapi/cpp/dev/file_chooser_dev.h
index ec0a6d5625ebfb96019299388e9158b779fe26ca..c2abc703073e71da43b57e9ee424d4272f8a64c9 100644
--- a/ppapi/cpp/dev/file_chooser_dev.h
+++ b/ppapi/cpp/dev/file_chooser_dev.h
@@ -12,7 +12,7 @@ namespace pp {
 
 class CompletionCallback;
 class FileRef;
-class Instance;
+class InstanceHandle;
 class Var;
 
 class FileChooser_Dev : public Resource {
@@ -42,7 +42,7 @@ class FileChooser_Dev : public Resource {
   /// TODO(darin): What if the mime type is unknown to the system?  The plugin
   /// may wish to describe the mime type and provide a matching file extension.
   /// It is more webby to use mime types here instead of file extensions.
-  FileChooser_Dev(const Instance* instance,
+  FileChooser_Dev(const InstanceHandle& instance,
                   PP_FileChooserMode_Dev mode,
                   const Var& accept_mime_types);